This invention relates to a system, including both apparatus and method, for controlling the operation of blasting apparatus to improve both the safety and ease of operation. In particular, it relates to a system for allowing the operator to control the apparatus by signals from a source in his grasp at the nozzle end of the hose from which the blast agent emerges under high pressure.
It has long been common practice to provide a switch, referred to as a dead man switch, at the operator's end of a high-pressure blasting hose to turn off the stream of blasting material if the operator releases his grip on the switch. Some dead man switches are fluid switches and are connected back to the pressure-controlling apparatus by fluid lines running alongside the main hose. One of the disadvantages of such switches is that it takes an appreciable length of time for the change in fluid conditions due to release of the switch to reach the valve or valves controlled by that fluid. If the switch is released as a result of some emergency condition, and not simply because the operator decided to stop blasting for any of the normal reasons, the time it takes for the change in fluid conditions to affect closure of the valve or valves can be a time when the high-pressure blasting material streams out of an uncontrolled hose. This is clearly a destructive and dangerous condition.
Other dead man switches are electrical and are connected back to the pressurizing apparatus by wires. Such switches are of the normally-open type and are held closed by the operator during a blasting operation. This allows current from a source to flow out through one wire to the switch and back to the control apparatus through the other wire. Any interruption in the current is perceived instantaneously at the control apparatus and begins to turn off the pressurized stream at once. Thus, the time taken for the stream actually to stop is much less than in the case of fluid-controlled switches.
However, electrical switches are subject to another kind of failure. The wires running along the hose are subject to severe wear due to the rough surfaces over which they are likely to be dragged, and the insulation on the wires sometimes wears off, allowing the wires to be short-circuited together, either by direct contact or by mutual contact with a conducting material. As far as current from the source is concerned, there is no difference between flowing through a dead man switch and flowing through a short circuit. The switch by-passed by a short circuit is cut out of the circuit without the knowledge of the operator or any of the safety personnel.
If the dead man switch were of the normally-closed type that had to be held open by the operator to blast a surface, the fact that wires connected to the switch can be broken by the rough treatment to which they are unavoidably exposed would remove the switch from the circuit with the same dangerous effect as short-circuiting the wires connected to a normally-open switch.
In addition to the types of failure that may affect the dead man switches currently in use, there are other aspects of the operation of blasting apparatus that must be controlled. In a typical blasting set-up, there is a large container of material, called blast agent, to be directed forcefully against a surface that is to be treated, such as for the removal of paint, rust, scale, or other materials. Air is compressed and forced as a high-pressure stream through a channel that usually runs under the container, which is commonly called a blast pot. Attached to the blast pot is a connection that allows the blast agent to enter the channel where it will be entrained by the high-pressure stream. The high-pressure stream, which now includes the blast agent, enters a flexible hose that carries it out to the blast site where it emerges from the nozzle as an abrasive stream that can be directed by the operator to strike any point on the surface to be blasted.
A control valve is normally provided between the point at which the pressurized stream of air enters the channel and the point at which the blast agent enters the stream. This valve is closed to stop the stream in response to release of the dead man switch. In addition, in order to cause the blast agent to flow uniformly into the channel carrying the high pressure stream, the space at the top of the blast pot is pressurized, such as by the same means that pressurizes the stream, so that both the outlet of the pot and the space above the blast agent are at the same pressure. This should allow the blast agent to flow into the stream as if the agent were under no pressure at all, except for the fact that moisture condenses in the pot and causes the blast agent to cake up.
The blast agent is kept as dry as possible prior to being put in the pot, and the moisture that reaches it there comes from the air fed into the top of the pot under high pressure. That air comes from the ambient air and always has some moisture in it. When it emerges from the relatively small pipe by which it reaches the upper part of the pot, it suddenly encounters an area of somewhat lower pressure, and this reduces the temperature of the air. As a result, moisture in it condenses, sometimes to the point of falling as rain inside a blast pot.
In order to remove a choking lump of blast agent at the outlet from the blast pot to the channel carrying the high-pressure stream, the control valve is turned off momentarily so that the only pressure on the blast agent in the pot is at the top, forcing the blast agent down into the channel. Heretofore, it has not been possible for the operator at the remote end of the hose to adjust the control valve, and someone else, such as a safety person has had to do the job.
The connection from the pot to the channel is normally not a simple junction but a blast agent valve of considerable complexity. Such valves are commonly not only capable of interdicting the flow of blast agent entirely but also of controlling the rate at which the blast agent can flow into the channel. In effect, the valve controls the size of the passageway from the pot into the channel. This permits the ratio of air to blast agent in the emerging stream to be changed by changing the setting of the blast agent valve. As in the case of adjusting the control valve, the usual practice has been to have someone else stationed at the pot to carry out such adjustments at the direction of the operator. While the inherent dangers in blasting require an additional person for the sake of safety, the difficulties of communication due to the noise, frequent poor visibility, and the fact that the operator is sometimes totally out of sight of the second person make it highly desirable that the operator be able to control both the stream and the blast agent by himself.
Dremann has disclosed a pneumatic blast agent valve and a system for controlling it in U.S. Pat. 4,075,789, but his control system, being pneumatic, is subject to the delay common in pneumatic dead man controls. While Dremann also, as a second embodiment, a manually operated form of his blast agent valve, his pneumatic system for modulating the size of the blast agent passageway cannot be used with that modification nor can it be used with other manually or mechanically operated blast agent valves.
It is also important for a blasting control system to be able, without degrading the response of the system to the normal operation of the dead man switch, to respond to a short-circuited cable or a broken cable automatically and to provide warnings related to specific malfunctions of the equipment.